What Is a Brother-Sister Controlled Group of Companies?

A brother-sister controlled group is an IRS classification that treats two or more businesses as a single employer for tax purposes when the same small group of owners — five or fewer individuals, estates, or trusts — holds more than 50% identical ownership across them. The businesses don’t have to work together, share customers, or operate in the same industry. What matters is who sits on the ownership side. Once the test is met, the companies share retirement plan compliance obligations, health coverage thresholds, and several tax deductions that would otherwise apply to each business on its own.

The Ownership Test That Creates the Group

The definition lives in Internal Revenue Code Section 1563(a)(2). Two or more corporations form a brother-sister controlled group when five or fewer people own more than 50% of each corporation, counting only the identical portion of each person’s ownership across all the corporations being tested.¹

The word “identical” carries most of the weight. The test does not ask whether the same people collectively hold a majority of each company. It asks how much of each person’s stake overlaps when you line up their ownership in every company side by side. Only the overlap counts, and the overlapping total has to exceed 50%.

This is different from a parent-subsidiary controlled group, where one corporation directly owns at least 80% of another. In a brother-sister structure, none of the companies owns any of the others. The link runs through the individuals sitting above all of them.

How to Calculate Identical Ownership

For each owner, take the lowest percentage that person holds in any of the companies being tested. Add those minimums together. If the total is more than 50%, the companies form a brother-sister controlled group.

A worked example with two companies and two owners:

Owner X holds 70% of Company A and 30% of Company B.
Owner Y holds 30% of Company A and 70% of Company B.

Owner X’s identical ownership is 30%, the lower of the two figures. Owner Y’s identical ownership is also 30%. Added together, that is 60%. Since 60% exceeds the 50% threshold, Company A and Company B are a brother-sister controlled group.¹

Change the numbers and the answer flips. If Owner X holds 80% of Company A and 5% of Company B, and Owner Y holds 20% of Company A and 95% of Company B, identical ownership is 5% plus 20%, or 25%. That is below the threshold. The overlap exists, but not enough of it.

When a Second 80% Test Applies

The original definition had two tests: the more-than-50% identical ownership test and a separate at-least-80% common ownership test. A 2004 amendment removed the 80% test from the core statutory definition used to apportion tax benefits among group members. Treasury regulations kept the stricter two-test version, however, for anything outside Sections 1561 through 1563 that references the controlled group definition. That includes the employee benefit rules under Sections 414(b) and 414(c).²

Under the two-test version, the same five or fewer owners must collectively own at least 80% of the voting power or total stock value of each corporation, and must also satisfy the more-than-50% identical ownership requirement. So a set of companies can be a brother-sister group for tax-benefit purposes yet fall outside the definition for retirement plan purposes. Most structures that clear the 50% test also clear 80%, but the edge cases matter when they come up.

Stock Owned by Family Members Counts Too

The IRS does not stop at shares each person directly holds. Under the constructive ownership rules in Section 1563(e), stock owned by certain family members and related entities is attributed to the person being tested. The point is to keep owners from sidestepping controlled group status by parking shares with relatives.¹

Family attribution works differently depending on the relationship. For minor children under 21, attribution runs automatically in both directions between parent and child. For adult children 21 or older, a parent is treated as owning the adult child’s stock only if the parent already owns more than 50% of the corporation, and the same threshold applies in reverse. Spouses are generally treated as owning each other’s stock, with a narrow exception when the non-owning spouse has no direct ownership, does not participate in managing the business, the company earns less than half its income from passive sources, and no restrictions favor the spouse or their minor children. Grandparents and grandchildren are attributed only when the person being tested already owns more than 50% of the corporation.

Beyond family, stock subject to an option is treated as owned by the option holder, and stock held by partnerships, estates, or trusts can be attributed to partners or beneficiaries with at least a 5% interest.¹

Not Only Corporations

Section 1563 itself is written for corporations. Section 414(c) extends controlled group treatment to all trades or businesses under common control, incorporated or not.³ LLCs, partnerships, and sole proprietorships can form brother-sister groups for employee benefit purposes on principles similar to the corporate rules. Owning an LLC and a corporation can trigger controlled group status just as readily as owning two corporations.

What Controlled Group Status Does to Retirement Plans

This is where the classification bites hardest. When companies form a controlled group, every employee across every member company is treated as if they all work for a single employer for retirement plan purposes.³

Coverage and Nondiscrimination Testing

Qualified plans like 401(k)s have to pass annual tests showing they don’t disproportionately benefit highly compensated employees. In a controlled group, the testing pool is every eligible employee across every member. Running separate tests for each business isn’t allowed. A 401(k) at one company that mostly benefits well-paid staff will often fail once the lower-paid workers at a sister company are pulled in.⁴

This surprises a lot of owners. A doctor who owns both a medical practice and a real estate LLC may find that the rental employees have to be counted in the practice’s 401(k) coverage testing.

Contribution Limits Apply Across the Group

The annual limit on total additions to a defined contribution account, $72,000 for 2026, applies across the entire controlled group rather than per company.⁵ An employee working for two brother-sister companies cannot receive $72,000 from each. The elective deferral limit, $24,500 for 2026, caps the total an employee can defer across all plans maintained by controlled group members.⁶

The deductible contribution limits under Section 404(a) also apply as if all members were a single employer, and the total deduction is allocated among members under IRS regulations.³

Health Coverage: ACA and COBRA

Controlled group status determines whether you’re an Applicable Large Employer under the Affordable Care Act. The employer mandate applies at 50 full-time employees, including full-time equivalents, and members’ workforces are combined for that count. The IRS treats companies related under Section 414 as a single employer for ALE purposes, so if the combined total hits the threshold, every entity in the group is subject to the employer shared responsibility provisions, including members that employ only a handful of people on their own.⁷

COBRA follows the same aggregation logic. Federal COBRA applies to employers with 20 or more employees, counted at the controlled group level. Two companies with 12 employees each wouldn’t trigger COBRA individually, but as a brother-sister group with 24 combined employees, both are covered.

Tax Deductions and Credits Have to Be Shared

Beyond employee benefits, several tax advantages that would apply separately to standalone businesses have to be shared among controlled group members. The goal is to stop owners from multiplying tax breaks by splitting one operation across several entities.

Section 179 Expensing

The Section 179 deduction, which lets businesses immediately expense qualifying equipment and property, is capped at $2,560,000 for tax years beginning in 2026, with a phase-out starting once total purchases exceed $4,090,000. For controlled group members, all component members are treated as a single taxpayer, and the dollar limit has to be apportioned among them.⁸ If Company A uses $2 million of the deduction, Company B can use no more than $560,000.

Accumulated Earnings Credit

Corporations can generally retain up to $250,000 in accumulated earnings, or $150,000 for certain personal service corporations, before the accumulated earnings tax applies. In a brother-sister controlled group, that credit has to be split among all members. The group can choose how under an apportionment plan, or divide it equally with no plan in place, but the total across members can’t exceed what a single corporation would receive.⁹

Qualified Business Income Deduction

The Section 199A qualified business income deduction, available to owners of pass-through businesses, allows an optional aggregation election for commonly controlled trades or businesses. Aggregation combines wages and capital across related businesses when calculating the deduction’s wage-and-capital limitation, which can sometimes yield a larger deduction than calculating each business separately.

Schedule O: What Corporate Members File

Every corporation that’s a member of a controlled group files Schedule O (Form 1120) with its income tax return. Schedule O reports how the group is apportioning its shared tax benefits, including the Section 179 deduction and the accumulated earnings credit. It has to be filed every year the corporation is a component member, even if the apportionment plan hasn’t changed from the prior year.⁹

If the group has a written apportionment plan agreed among members, the plan controls the allocation. Members don’t have to split benefits equally, and different benefits can be allocated in different proportions. With no written plan, the IRS requires equal division among members. Each member keeps a signed copy of the agreement in its records; the agreement itself is not attached to any member’s tax return.⁹

1
Office of the Law Revision Counsel. 26 USC 1563 – Definitions and Special Rules
2
eCFR. 26 CFR 1.1563-1 – Definition of Controlled Group of Corporations and Component Members and Related Concepts
3
Office of the Law Revision Counsel. 26 USC 414 – Definitions and Special Rules
4
Internal Revenue Service. Chapter 7 Controlled and Affiliated Service Groups
5
Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
6
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
7
Internal Revenue Service. Determining if an Employer Is an Applicable Large Employer
8
Office of the Law Revision Counsel. 26 USC 179 – Election to Expense Certain Depreciable Business Assets
9
Internal Revenue Service. Instructions for Schedule O (Form 1120) Consent Plan and Apportionment Schedule for a Controlled Group